Narrow bandwidth laser sources are very important for optical fiber telecommunication applications. The core technology of DWDM is to increase the capacity of single mode fiber by using multiple wavelengths to carry the information. The center wavelength stability of laser sources is very critical for DWDM optical systems. Narrow bandwidth laser sources are very important devices for instrumentation, sensor, biomedical, metrology and telecommunication applications. Solid-state lasers have advantages in compactness and efficiency over other types. To achieve a single transverse mode laser, you can use different techniques to suppress the high order transverse modes.
Fiber Bragg gratings were first used by Brian F. Ventrudo (U.S. Pat. No. 5,715,263) to stabilize the intensity and frequency fluctuations of laser diodes. In Brian F. Vetrudo invention, an optical lens is used to couple the laser diode with optical fiber which contains the fiber Bragg grating. Jean-Marc Verdiell et al. have used fiber Bragg gratings with high reflectivity back facet laser diodes to form an external cavity for suppression of longitudinal mode hops and compensation of wavelength shift (U.S. Pat. No. 5,870,417). A recent invention by Dmitri V. Kuksenkov et al. uses fiber gratings to define the end of the optical cavity for discriminating against the lasing of higher-order transverse modes in the multi-mode gain region (U.S. Pat. No. 6,625,182). There are some disadvantages for these inventions which use fiber Bragg gratings as external cavities to suppress longitude and lateral mode hops:                1. The light from laser diodes, solid state lasers or gain mediums must be coupled into the optical fiber with high energy loss;        2. The fiber Bragg gratings are usually not located at the fiber end, but some distance from the fiber end increasing the length of the optical cavity, which makes it difficult to modulate the laser diode at a high frequency.        3. There are many applications that need free space laser diodes and do not need light to be coupled into optical fibers, which limits the use of Fiber Bragg gratings as external cavities.        
Others have used volume holographic gratings as external cavities to reduce laser bandwidth and to stabilize laser operation wavelength (U.S. Pat. No. 5,691,989). Volume holographic gratings are of small size (around 1 mm cubic) and they can be packaged inside a TO-Can of a laser diode. A laser diode with a volume holographic grating external cavity still has a free space beam. Several companies now manufacture laser diodes with volume holographic external cavities. There are also some drawbacks for volume holographic gratings as laser diodes external cavities:                1. High manufacturing cost and complex manufacturing process of the volume holographic gratings are the main obstacles to the widely use of volume holographic gratings;        2. Volume holographic gratings can not stand high power applications;        3. Long and short period volume holographic gratings are difficult to be manufactured.        
It can be seen that the present invention provides a simple solution of external cavity for free space laser diodes and solid state lasers which overcomes the problems met with the two approaches mentioned above.